Various antennas including linear antennas are known conventionally. Any of these reception antennas has an operational impedance R.sub.o, so that a feeder having a characteristic impedance R.sub.o equal to the operational impedance R.sub.o is connected to the antenna to lead received electric waves to a receiver.
However, since the real part of the operational impedance R.sub.o itself is a source of thermal noise, the received signal would be covered with the thermal noise if there is no reception field strength which exceeds the thermal noise. Thus the received signal is available even if it is amplified in the subsequent stages to whatever degree. Namely, there is a minimum limit to the reception field strength.
It is an object of the present invention to provide a small relatively wide-band active antenna which is capable of receiving in principle any small electric waves below the minimum limit to the reception field strength